1. Field of the Invention
The present invention relates to a centrifugal compressor capable of compressing a fluid, in particular to a centrifugal compressor structure with impellers adapted to convert kinetic energy generated from a motor into pressure energy, in which each of the impellers adjusts an axial load, thereby appropriately adjusting a load applied to a thrust bearing.
2. Description of the prior Art
Generally, compressors are adapted to convert mechanical energy generated by a motor into pressure energy, thereby increasing the pressure of a fluid. In particular, centrifugal compressors, to which the present invention relates, are adapted to conduct a compression for a fluid by use of the rotating force of an impeller while sucking the fluid in an axial direction, and discharging the sucked fluid in a centrifugal direction. Typically, such centrifugal compressors include multiple stages so that they operate in a multi-stage compression. In particular, two-stage centrifugal compressors including two stages of compression are mainly used.
Such centrifugal compressors are mainly used in air conditioners and specific military equipment. In accordance with the capacity of a fluid to be compressed, centrifugal compressors are classified into those of a large capacity and those of a small capacity.
FIG. 1 is a sectional view illustrating the structure of a conventional two-stage centrifugal compressor.
Referring to FIG. 1, the conventional two-stage centrifugal compressor is of a back-to-back type in which impellers face each other at their back surfaces. Now, the structure of this centrifugal compressor will be described in conjunction with FIG. 1.
As shown in FIG. 1, the centrifugal compressor includes a motor case 1 having a desired shape to receive units including a motor while isolating those units from the outside. The motor, which is denoted by the reference numeral 2, is also included in the centrifugal compressor. The motor 2 is disposed in the motor case 1 and adapted to convert electric energy into mechanical kinetic energy.
The centrifugal compressor also includes a drive shaft 3 axially coupled to the motor 2 to rotate along with the drive shaft 3. A pair of impellers, that is, a first impeller 4 and a second impeller 5, are coupled to opposite ends of the drive shaft 3, respectively, and convert a rotating movement of the drive shaft 3 into kinetic energy to be applied to a fluid. The centrifugal compressor further includes thrust bearings 6 disposed at a portion of the drive shaft 3 in the vicinity of a first end of the drive shaft 3. The thrust bearings 6 are adapted to gently support a thrust load axially applied to the drive shaft 3.
A pair of radial bearings 7 and 8 are respectively disposed at portions of the drive shaft 3 in the vicinity of opposite ends of the drive shaft 3. The radial bearings 7 and 8 are adapted to radially support the drive shaft 3, thereby gently supporting a radial load. A pair of bearing plates, that is, a first bearing plate 9 and a second bearing plate 10, each interposed between the motor case 1 and an associated one of the radial bearings 7 and 8, are adapted to allow the associated radial bearing to be supported by the motor case 1.
A bearing cover 11 is fitted around the first end of the drive shaft 3 installed with the trust bearings 6 while being fitted in a first end of the motor case 1 corresponding to the end of the drive shaft 3. The bearing cover 11 seals the interior of the motor case 1. A pair of diffusers, that is, a first diffuser 12 and a second diffuser 13, are arranged at respective discharge ends of the impellers 4 and 5 in order to convert kinetic energy, possessed in the fluid discharged at a high velocity from the impellers 4 and 5, into pressure energy.
A first volute case 14 is mounted to the outside of the first diffuser 12. The first volute case 14 has a desired shape to collect the fluid discharged in a compressed state from the first diffuser 12 while reducing the pressure energy possessed in the discharged fluid. A connecting tube 15 is connected at one end thereof to the first volute case 14 to guide the fluid discharged from the first volute case 14 toward the second impeller 5. A second volute case 16 is mounted to the outside of the second diffuser 13. The second volute case 16 is connected to the other end of the connecting tube 15 to temporarily collect the fluid emerging from the connecting tube 15, and then being compressed again while passing sequentially through the second impeller 5 and the second diffuser 13.
The centrifugal compressor further includes a plurality of uniformly-spaced fluid passages 17. The fluid passages 17 extend axially through the second bearing plate 10 and are adapted to allow the high pressure fluid collected in the second volute case 16 to be discharged from the second volute case 16. A motor chamber 18 is defined between the first and second bearing plates 9 and 10 in the interior of the motor case 1. The motor chamber 18 receives the fluid discharged through the fluid passages 17 and allows the received fluid to stay temporarily therein while cooling the motor 2.
The centrifugal compressor also includes a labyrinth seal 19 formed at a surface of the bearing cover 11 contacting the drive shaft 3. The labyrinth seal 19 is adapted to prevent the high pressure fluid filled in the motor chamber 18 from being leaked outwardly from the motor chamber 18. A discharge tube 20 is connected at one end thereof to a desired portion of the motor case 1, while communicating with the motor chamber 18 and adapted to discharge the high pressure fluid from the motor chamber 18. A suction tube 21 is connected to the first volute case 14 upstream from the first impeller 4.
The operation of the two-stage centrifugal compressor will now be described in brief. A fluid to be compressed is introduced into the centrifugal compressor via the suction tube 21. The introduced fluid is primarily compressed by the first impeller 4, and then forced to pass through the first diffuser 12, so that it is highly pressurized. The high pressure fluid is then collected by the first volute case 14 without any loss of pressure. The collected fluid is introduced into the second impeller 5 which, in turn, secondarily compresses the fluid. The secondarily compressed fluid is then further compressed to a higher pressure while passing through the second diffuser 13, and then collected in the second volute case 16. The high pressure fluid is then introduced into the motor chamber 18 via the fluid passages 17, so that it cools the motor 2 heated to a high temperature. After cooling the motor 2, the fluid is outwardly discharged from the motor chamber 18 via the discharge tube 20.
During the above operation, considerably high pressure is applied to the first and second impellers 4 and 5. Due to such high pressure, a high load is applied to the thrust bearings 6. Now, effects resulting from such a load will be described in detail.
FIG. 2 is a plan view illustrating one of the impellers used in the above mentioned conventional centrifugal compressor, that is, the impeller 4.
Referring to FIG. 2, the impeller 4 has a structure in which a plurality of blades 4b are mounted around a cylindrical hub 4a. Once an external fluid is axially introduced into the center of the impeller 4 during a rotation of the impeller 4, it is then forced to move in a centrifugal direction along the blades 4b conducting a rotation. As the fluid moves in the centrifugal direction, it possesses kinetic energy, so that it is converted into a fluid having high energy, that is, a high pressure fluid flowing at a high velocity.
FIG. 3 is a sectional view illustrating the impeller 4 used in the conventional centrifugal compressor.
As shown in FIG. 3 and mentioned above, the impeller 4 includes the hub 4a forming a body of the impeller 4. The blades 4b are mounted to a front surface of the hub 4a. The fluid, which has been changed into a high pressure fluid flowing at a high velocity while passing the blades 4b, is further compressed at the back side of the impeller 4, so that an increased axial load is applied to the impeller 4.
The load applied to the impeller 4 due to the above mentioned operation is schematically illustrated in FIG. 4.
As apparent from FIG. 4, the fluid exerting its pressure on the impeller 4 strongly pushes the impeller 4 in a forward direction while slightly pushing the impeller 4 in a backward direction because the fluid reaching the back surface of the impeller 4 after passing the blades 4b has a pressure considerably higher than the pressure of the fluid exerting on the front surface of the impeller 4. As a result, the impeller 4 generates a force urging it in a direction from the back surface thereof to the front surface thereof. Such an urging force is also generated at the impeller 5. These pushing forces are vector-summed, thereby leaving a force F which is, in turn, applied to the drive shaft 3.
At this time, the fluid pressures respectively radially applied to the impellers 4 and 5 disappear because they are offset by each other by virtue of the symmetrical arrangement of the impellers 4 and 5.
In the above mentioned configuration, the axial load applied to each impeller is supported by the thrust bearings (denoted by the reference numeral 6 in FIG. 1). That is, the axial load is continuously applied to the thrust bearings 6. As a result, the thrust bearings 6 may be eventually damaged.
In order to solve this problem, a method has been proposed in which respective outer diameters of the impellers 4 and 5 are adjusted, based on a difference between the pressures respectively applied to the impellers 4 and 5 arranged at opposite ends of the drive shaft 3, to offset axial loads respectively applied to the impellers 4 and 5. However, such an adjustment for the diameters of the impellers 4 and 5 results in an undesirable variation in compression ratio. For this reason, there is a difficulty in determining an appropriate compression ratio when the centrifugal compressor is designed.
Therefore, the present invention has been made to overcome the above mentioned problems, and an object of the present invention is to provide a centrifugal compressor structure with impellers, in which the axial load generated from each of the impellers respectively coupled to opposite ends of a drive shaft can be adjusted without any reduction of the outer diameter of the impeller, so that errors generated during the manufacture of the compressor are reduced, thereby allowing the compressor to be more conveniently manufactured.
In accordance with the present invention, this object is accomplished by providing a centrifugal compressor structure including at least one impeller, the impeller comprising: a hub coupled to a drive shaft and adapted to receive a rotating force from a motor via the drive shaft so that it rotates; a plurality of blades provided at a front surface of the hub and adapted to receive a rotating force from the hub, thereby compressing an external fluid while forcing the fluid to flow from an upstream end of the hub to a downstream end of the hub; and a plurality of uniformly-shaped pressure attenuating grooves provided at an outer peripheral edge of the hub and adapted to reduce an axial load applied to the impeller.
The pressure attenuating grooves are formed while having no influence on the blades. These pressure attenuating grooves serve to reduce a load resulting from a high hydraulic pressure exerted on the back surface of the impeller.